The present invention refers to rotors for in-out electric motors and to a process for their manufacture.
In-out electric motors, that is, motors built with the stator on the inside and the rotor on the outside, are known. In such motors the rotor rotates centered about the stator because it is rigidly connected to a shaft which in turn rotates in one or more supports centered with respect to the geometric shape of the stator and contained in the central part of the stator itself or in an apposite tube or housing rigidly connected therewith.
The coaxial relationship of the shaft with respect to the outside of the stator is better, the greater the axial extension of the bearing of the shaft, or the number of the shaft bearings if there are a plurality of them and provided that they are positioned at a considerable mutual distance. This is advantageous from other viewpoints, in that it permits the use of simple bearings, reduces the forces acting thereon, improves lubrication, and so on. From the electrical view point, especially in small, and particularly in two-pole in-out motors, it is desirable that the bore of the stator through which the shaft passes be as small as possible because the entire core of the stator is an active portion of ferromagnetic material. To obtain this, the bearings, which occupy a considerable radial space, have to be shifted axially to the outside of the stator.
These and other structural exigencies of in-out electric motors are not adequately met in the art, and the rotor structures as well as the processes for making the same are not satisfactory.
In the known motors the rotor is provided at one of its ends with a hub which is connected to the shaft which rotates centered with respect to the geometric shape of the stator. This is obtained, in known processes, by casting together with a short-circuit ring of the rotor a cup-like shield which extends until it connects with a hub and which actually is usually cast as solid therewith. Since the material most adapted for making short-circuit rings is pure aluminium, which however has a low elastic limit and a low elastic modulus and is easily deformable, both the shield and hub have to made rather thick. Thus the hub itself acquires considerable axial and radial dimensions which subtract space from the bearing and force it to be located more internally with respect to the motor, thus decreasing the distance from the other bearing and occupying a volume which would be much more advantageously occupied by ferromagnetic material. The shield, with its considerable thickness, increases the axial dimension of the motor proper at the zone where this dimension is given by the axial dimension of the windings plus the thickness of the shield, which is in contrast with the requirement, highly desired in this type of motor, of containing the axial dimension within the narrowest possible limits. The use of materials mechanically more resistant than pure aluminium, such as for instance aluminium-silicium alloys, has not solved the problem because the greater electric resistance of such materials makes it necessary to employ greater cross-sections for the rotor cavities, thus requiring a greater amount of cast material, and increases the dimensions of the motor because the ferromagnetic material substracted by the enlargment of the cavities has to be integrated in some way.
Further, when apertures for the inlet of air which ventilates the inside of the motor have to be provided in the shield near the hub, the thickness of the shield must be further increased to compensate for the loss of rigidity due to such apertures, thus further increasing the axial dimension of the motor.
In another known construction, the hub connected to the shaft is in turn connected to a rigid metal disc provided with radial projections which are incorporated as inserts in the cast of the adjacent short-circuit ring. This structure too, however, is not satisfactory because it does not fully satisfy the aforementioned structural conditions, and in particular, the space taken up by the hub, which still exists, does not permit the maximum spacing of the bearings of the rotor shaft associated with the maximum containment of the axial dimensions of the motor. Further, the known processes for making the rotor do not provide a perfect centering of the shaft with respect to the inner rotor surface without additional finishing operations, which are operatively and economically burdensome.